Beginner's Guide to Base Metals & Alloying
Iron (Fe) is a relatively soft, malleable, and ductile metal. It's strongly magnetic and rusts easily in moist air. Pure iron has limited engineering applications due to its low strength.
Steel is an alloy of iron and carbon, typically with a carbon content between 0.2% and 2.1% by weight. Carbon is the primary hardening element. Steels offer a vast range of mechanical properties and are the most widely used metallic materials in construction and engineering.
Common Alloying Elements in Steel and Their Effects:
- Carbon (C): The most crucial alloying element. Increases hardness, tensile strength, and responsiveness to heat treatment. Decreases ductility and weldability.
- Manganese (Mn): Increases strength, hardness, and hardenability. Acts as a deoxidizer and desulfurizer, improving hot workability.
- Chromium (Cr): Increases hardness, toughness, and wear resistance. Crucially, it imparts corrosion resistance (essential for stainless steels, typically >10.5% Cr). Improves high-temperature strength.
- Nickel (Ni): Increases strength, toughness (especially at low temperatures), and corrosion resistance. Important in austenitic stainless steels.
- Molybdenum (Mo): Increases strength, hardness, hardenability, and toughness, especially at elevated temperatures (creep resistance). Enhances corrosion resistance, particularly against pitting in stainless steels.
- Silicon (Si): Acts as a deoxidizer. Increases strength and hardness. In cast irons, promotes graphite formation.
- Vanadium (V): Increases strength, toughness, and wear resistance. Promotes fine grain structure.
Aluminum (Al) is a lightweight, silvery-white, non-magnetic, and ductile metal. It has excellent corrosion resistance due to the formation of a passive oxide layer (passivation). It's a good thermal and electrical conductor. Pure aluminum is relatively soft and not very strong, so it's often alloyed.
Common Alloying Elements in Aluminum and Their Effects:
- Silicon (Si): Improves fluidity and reduces solidification shrinkage, making it excellent for casting alloys. Increases strength and hardness, and wear resistance.
- Copper (Cu): Significantly increases strength and hardness, especially through heat treatment (precipitation hardening). Can reduce corrosion resistance and weldability.
- Magnesium (Mg): Increases strength through solid solution strengthening and improves strain hardening ability. When combined with silicon (as Mg₂Si), allows for heat treatment (6xxx series alloys). Generally improves corrosion resistance.
- Manganese (Mn): Increases strength somewhat through solution strengthening. Improves strain hardening and controls grain structure.
- Zinc (Zn): When combined with magnesium (and sometimes copper), produces the highest strength heat-treatable aluminum alloys (7xxx series).
- Titanium (Ti): Used as a grain refiner, improving mechanical properties and preventing cracking in castings and welds.
Titanium (Ti) is a strong, lightweight, corrosion-resistant metal with a silver color. It has a very high strength-to-density ratio. Its excellent corrosion resistance is due to a stable, protective oxide layer. Titanium exists in two main crystallographic forms (alpha and beta), which influences alloying behavior.
Common Alloying Elements in Titanium and Their Effects:
- Aluminum (Al): Primarily an alpha stabilizer. Increases strength (both at room and elevated temperatures) and lowers density. Too much Al can lead to embrittlement.
- Vanadium (V): A beta stabilizer. Improves hardenability and strength. Ti-6Al-4V is the most common titanium alloy, where Vanadium contributes significantly to its heat treatability and strength.
- Molybdenum (Mo): A strong beta stabilizer. Increases strength, hardenability, and high-temperature properties.
- Chromium (Cr): A beta stabilizer. Similar effects to Molybdenum, enhances strength and hardenability.
- Iron (Fe): A beta stabilizer. Can increase strength but may reduce ductility if present in high amounts or as undesirable phases.
- Oxygen (O), Nitrogen (N), Carbon (C): Interstitial elements. Small amounts can significantly increase strength and hardness but drastically reduce ductility and toughness. Controlled additions are used in some CP (Commercially Pure) grades.
Copper (Cu) is a ductile metal with very high thermal and electrical conductivity. It's reddish-brown, relatively soft, and has good corrosion resistance in many environments. Pure copper is widely used for electrical wiring and plumbing.
Brasses are primarily alloys of copper and zinc. Bronzes are primarily alloys of copper, usually with tin as the main additive, but the term is also used for alloys with other elements like aluminum or silicon.
Common Alloying Elements in Copper and Their Effects:
- Zinc (Zn): Forms Brass. Increases strength, hardness, and ductility (up to ~35% Zn). Improves castability and machinability (especially with lead additions). Reduces cost compared to pure copper. Higher Zn content can decrease corrosion resistance (dezincification).
- Tin (Sn): Forms Bronze. Significantly increases strength, hardness, and wear resistance. Improves corrosion resistance. Reduces electrical conductivity more than zinc.
- Aluminum (Al): Forms Aluminum Bronze. Provides high strength, excellent corrosion resistance (especially in seawater), and good wear resistance.
- Silicon (Si): Forms Silicon Bronze. Increases strength and corrosion resistance. Improves castability and weldability.
- Nickel (Ni): Forms Copper-Nickel alloys (Cupronickels). Greatly enhances corrosion resistance, especially in seawater and against biofouling. Improves strength at elevated temperatures.
- Lead (Pb): Added to brasses and bronzes (typically up to ~3%) to significantly improve machinability by acting as a chip breaker. Reduces ductility and strength.
- Phosphorus (P): Often used as a deoxidizer in copper alloys. Can increase strength and hardness but significantly reduces electrical conductivity.
Nickel (Ni) is a silvery-white, hard, ductile, and ferromagnetic metal. It exhibits excellent corrosion resistance in many environments, especially alkaline solutions. Pure nickel is used for plating, coinage, and specialized chemical equipment.
Nickel-based Superalloys are complex alloys designed for outstanding strength, creep resistance, and oxidation/corrosion resistance at very high temperatures (typically above 650°C / 1200°F). They are essential in gas turbines, jet engines, and other extreme environments.
Common Alloying Elements in Nickel Alloys/Superalloys and Their Effects:
- Chromium (Cr): Essential for high-temperature oxidation and corrosion resistance (forms a stable Cr₂O₃ protective scale). Contributes to solid solution strengthening. Key in alloys like Inconel.
- Molybdenum (Mo): Provides significant solid solution strengthening. Enhances resistance to pitting and crevice corrosion. Important in alloys like Hastelloy.
- Cobalt (Co): Can increase strength at high temperatures and improve creep resistance. Often used in conjunction with other elements.
- Aluminum (Al) & Titanium (Ti): These are key precipitation hardening elements in many nickel superalloys. They form fine, coherent gamma prime (γ') precipitates that dramatically increase high-temperature strength and creep resistance.
- Iron (Fe): Can be a base element (e.g., Incoloy series) or an addition to nickel-based alloys to modify properties and reduce cost. Often improves weldability.
- Niobium (Nb) / Columbium (Cb) & Tantalum (Ta): Form carbides and contribute to precipitation hardening (gamma double prime γ'' in Inconel 718). Improve creep strength and weldability in some alloys.
- Tungsten (W): Potent solid solution strengthener at high temperatures. Increases creep resistance.
- Carbon (C): Forms carbides with elements like Cr, Mo, Ti, Nb, W. Carbides can strengthen grain boundaries or contribute to wear resistance, but their morphology and location must be carefully controlled to avoid embrittlement.
Common Material Properties Terminology
- Yield Strength (MPa)
- The stress at which a material begins to deform plastically (permanently). Before this point, deformation is elastic (temporary). Measured in Megapascals (MPa). [1, 3]
- Tensile Strength (MPa)
- The maximum stress a material can withstand while being stretched or pulled before necking (local reduction in cross-sectional area) begins, leading to fracture. Measured in Megapascals (MPa). [1, 2]
- Modulus of Elasticity (Young's Modulus) (GPa)
- A measure of a material's stiffness or resistance to elastic deformation under tensile or compressive stress. It's the ratio of stress to strain in the elastic region. Measured in Gigapascals (GPa). [1, 5]
- Density (g/cm³)
- The mass of a material per unit volume. Commonly expressed in grams per cubic centimeter (g/cm³). [1, 2]
- Hardness (e.g., HB, HRC)
- A measure of a material's resistance to localized plastic deformation, such as indentation, scratching, or abrasion. Common scales include Brinell Hardness (HB) and Rockwell Hardness (HRC). [1, 3]
- Ductility
- The ability of a material to undergo significant plastic deformation (e.g., be stretched, pulled, or drawn into a wire) before fracturing. [1, 2]
- Toughness
- The ability of a material to absorb energy and plastically deform before fracturing. It represents a combination of strength and ductility. [1, 3]
- Hardenability (primarily for Steel)
- The ability of a steel to be hardened by heat treatment, specifically the depth to which martensite can be formed when quenched from austenitizing temperature. [1, 2]
- Creep Resistance
- A material's ability to resist slow, gradual deformation (creep) under constant stress, especially at elevated temperatures over extended periods. [1, 5]
- Fatigue Resistance / Endurance Limit
- A material's ability to withstand repeated cycles of stress or strain without failing. The endurance limit (or fatigue limit) is the stress level below which a material can theoretically withstand an infinite number of loading cycles. [1, 2]
- Notch Sensitivity
- The extent to which the presence of a notch, crack, or other stress concentrator reduces the strength or fatigue life of a material. Brittle materials are generally more notch-sensitive. [1, 2]
- Corrosion Resistance
- The ability of a material to withstand degradation and chemical breakdown due to reactions with its environment (e.g., oxidation, rusting, pitting). [1, 3]
- Thermal Conductivity (W/m·K)
- A measure of a material's ability to conduct or transfer heat. Expressed in Watts per meter-Kelvin (W/m·K). [1, 2]
- Electrical Conductivity (% IACS or S/m)
- A measure of how well a material conducts an electric current. Often expressed as a percentage of the International Annealed Copper Standard (% IACS) or in Siemens per meter (S/m). [1, 5]
- IACS (International Annealed Copper Standard)
- A standard where the conductivity of annealed copper at 20°C is defined as 100% IACS. Other materials' conductivities are expressed relative to this. [1, 2]
- Passivation
- A process of treating or coating a metal to reduce its chemical reactivity. In stainless steels, it involves the formation of a protective, passive oxide layer (typically chromium oxide) on the surface, enhancing corrosion resistance by removing free iron. [1, 2]
- Machinability
- The ease with which a metal can be cut or shaped by machining processes, resulting in a good surface finish and tool life. [1, 2]
- Weldability
- The ability of a material to be welded under given conditions to form a sound joint that performs satisfactorily in its intended service. [1, 2]
- Heat Treatment
- Controlled heating and cooling processes applied to metals to alter their microstructure and, consequently, their physical and mechanical properties (e.g., hardness, strength, ductility). [3, 4]
- Annealing: A heat treatment process that alters a material's microstructure to typically increase its ductility, reduce hardness, and relieve internal stresses, making it more workable. [2, 4]
- Quenching: Rapid cooling of a heated metal, often by immersion in water, oil, or air, to achieve specific microstructures like martensite for increased hardness. [2, 3]
- Tempering: A heat treatment process applied after quenching to reduce brittleness and relieve internal stresses, usually by heating to a temperature below the lower critical temperature, holding, and then cooling. [2, 3]
- Aging (Age Hardening): A heat treatment that induces precipitation of fine particles within a metal's microstructure over time, either at room temperature (natural aging) or elevated temperatures (artificial aging), to increase strength and hardness. See Precipitation Hardening. [3, 5]
- Solution Treatment (Solution Annealing): Heating an alloy to a suitable temperature to dissolve alloying elements into a solid solution, followed by rapid cooling to retain this state. This prepares the material for subsequent aging or other treatments. [2, 5]
- Precipitation Hardening: A strengthening mechanism involving the formation of fine, uniformly dispersed secondary phase particles (precipitates) within the primary phase of a metal alloy during heat treatment (aging). [3, 5]
- Work Hardening (Strain Hardening)
- The strengthening of a metal by plastic deformation (e.g., rolling, drawing, bending) at a temperature below its recrystallization point. This increases hardness and strength but usually reduces ductility. [1, 2]
- Solid Solution Strengthening
- A strengthening mechanism in metals achieved by adding atoms of one element (solute) to the crystal lattice of another element (solvent), forming a solid solution. The solute atoms distort the lattice, impeding dislocation movement. [1, 2]
- Interstitial Strengthening
- A type of solid solution strengthening where small solute atoms (e.g., carbon, nitrogen) occupy the interstitial sites (spaces between solvent atoms) in the crystal lattice, causing significant lattice distortion and impeding dislocation movement. [1, 4]
- Sensitization (in Stainless Steel)
- A phenomenon in some stainless steels where chromium carbides precipitate at grain boundaries when exposed to elevated temperatures (approx. 425-815°C). This depletes chromium in adjacent regions, making the steel susceptible to intergranular corrosion. [1, 3]
- Stress Corrosion Cracking (SCC)
- The initiation and growth of cracks in a material due to the combined action of tensile stress (applied or residual) and a specific corrosive environment. [1, 2]
- Galvanic Corrosion (Bimetallic Corrosion)
- An electrochemical process where one metal corrodes preferentially when in electrical contact with a different metal (the cathode) in the presence of an electrolyte. The more active metal becomes the anode and corrodes. [1, 2]
- Hydrogen Embrittlement
- A reduction in the ductility and toughness of a metal due to the absorption and diffusion of atomic hydrogen, which can lead to premature failure under stress. High-strength steels are particularly susceptible. [1, 2]
- Dezincification
- A selective leaching corrosion process where zinc is preferentially removed from brass alloys, leaving behind a porous, copper-rich, and weakened structure. [1, 2]
- Temper Embrittlement
- A reduction in the toughness of certain steels when tempered or held within a specific temperature range (typically 345-575°C), often due to the segregation of impurity elements to grain boundaries. [1, 2]
- Phases (e.g., Ferrite, Austenite, Martensite, Bainite)
- Distinct, homogeneous regions within a material that have a specific crystal structure and composition. Common phases in steel include:
- Ferrite: A body-centered cubic (BCC) iron phase, relatively soft and ductile, magnetic. [1]
- Austenite: A face-centered cubic (FCC) iron phase, typically stable at high temperatures, non-magnetic, can dissolve more carbon than ferrite.
- Martensite: A very hard and brittle body-centered tetragonal (BCT) phase formed by rapid cooling (quenching) of austenite. [1, 3]
- Bainite: A microstructure consisting of ferrite and cementite (iron carbide) that forms at temperatures between those for pearlite and martensite. It offers a combination of strength and toughness. [1, 2]
- Intermetallic Compound
- A phase in an alloy system with a distinct chemical formula and crystal structure, formed by two or more metallic elements (and sometimes non-metals) in fixed stoichiometric proportions. Often hard and brittle. [1, 3]
- Alloying Element & Base Metal
- Base Metal: The primary metal in an alloy (e.g., iron in steel, aluminum in aluminum alloys). [1, 2] Alloying Element: An element intentionally added to a base metal to modify its properties. [1, 2]
- Configurational Entropy (in HEAs)
- A measure of the randomness or disorder in the atomic arrangement of an alloy due to the mixing of multiple principal elements. In High Entropy Alloys (HEAs), high configurational entropy is thought to stabilize simple solid solution phases. [1, 5]
Carbon & Alloy Steels
| Material | Common Equivalents | Typical Forms | Yield (MPa) | Tensile (MPa) | Modulus (GPa) | Density (g/cm³) | Hardness | Cost Tier | Details |
|---|---|---|---|---|---|---|---|---|---|
| A36 Carbon Steel | UNS K02600, ASTM A36, EN S275JR | Plate, Shapes (Beams, Angles, Channels), Bar | 250 | 400-550 | 200 | 7.85 | ~120-160 HB | $ |
Key Performance:
Primary Applications:Structural beams (high-rise, bridges), general fabrication, plates, machinery parts, low-stress components. Critical Limitations:Corrosion in marine/chemical environments without coating. Limited to ~400°C service temperature due to strength loss. Processing:Readily weldable by common methods, good machinability. Not typically heat-treated for strength (used as-rolled). |
| 4140 Alloy Steel | UNS G41400, AISI 4140, EN 42CrMo4 (1.7225) | Bar, Rod, Forging, Tube, Plate | 415 (Ann) - 655+ (Q&T) | 655 (Ann) - 1020+ (Q&T) | 205 | 7.85 | ~197 HB (Ann), 28-34 HRC (Q&T) | $$ |
Key Performance:
Primary Applications:Automotive axles, crankshafts, medium-duty gears, bolts, couplings, spindles, tool holders. Critical Limitations:Requires proper heat treatment for optimal properties. Susceptible to temper embrittlement if not carefully processed. Not ideal for highly corrosive environments without protection. Processing:Responds well to heat treatment (quenching and tempering). Machinable. Weldable with pre/post heat treatment. Can be nitrided for surface hardness. |
| 4340 Alloy Steel | UNS G43400, AISI 4340, EN 34CrNiMo6 (1.6582) | Bar, Rod, Forging, Plate, Tube | 470 (Ann) - 1515+ (Q&T) | 745 (Ann) - 1895+ (Q&T) | 205 | 7.85 | ~217 HB (Ann), 35-55 HRC (Q&T) | $$$ |
Key Performance:
Primary Applications:Aircraft landing gear, high-stress shafts and gears, military ordnance, connecting rods, structural parts requiring high strength and toughness. Critical Limitations:Notch-sensitive, requires careful design and heat treatment to avoid embrittlement. Prone to hydrogen embrittlement if improperly plated. Difficult to weld. Processing:Deep hardening. Requires specific heat treatments (austenitizing, quenching, tempering) for optimal properties. Weldable only with stringent procedures. |
Stainless Steels
| Material | Common Equivalents | Typical Forms | Yield (MPa) | Tensile (MPa) | Modulus (GPa) | Density (g/cm³) | Hardness | Cost Tier | Details |
|---|---|---|---|---|---|---|---|---|---|
| 304 SS (Austenitic) | UNS S30400, AISI 304, EN 1.4301, JIS SUS304 | Sheet, Plate, Bar, Tube, Pipe, Wire, Fittings, Casting | 205-310 | 515-620 | 193-200 | 8.0 | ~85 HRB (Annealed) | $$ |
Key Performance:
Primary Applications:Food processing equipment (tanks, piping), architectural trim, kitchen sinks, cutlery, brewery equipment, chemical tanks (mild service), exhaust systems. Critical Limitations:Chloride stress corrosion cracking (SCC) above ~60°C. Sensitization can reduce corrosion resistance at welds. Poor resistance to reducing acids. Processing:Non-hardenable by heat treatment. Strength increased by cold work. Excellent formability and ductility. Annealing restores ductility after cold work. |
| 316 SS (Austenitic) | UNS S31600, AISI 316, EN 1.4401/1.4436, JIS SUS316 | Sheet, Plate, Bar, Tube, Pipe, Wire, Fittings, Casting | 205-310 | 515-620 | 193-200 | 8.0 | ~85 HRB (Annealed) | $$$ |
Key Performance:
Primary Applications:Marine hardware (boat fittings, propellers), pharmaceutical equipment, chemical processing (tanks, pipes for more aggressive media), food processing, medical implants, pulp & paper industry. Critical Limitations:Chloride SCC above ~60°C, though more resistant than 304. Galvanic corrosion with aluminum, carbon steel. More expensive than 304. Processing:Non-hardenable by heat treatment. Cold work increases strength. Good formability. Annealing restores ductility. |
| 17-4 PH SS (Precipitation Hardening) | UNS S17400, AISI 630, EN 1.4542 | Bar, Rod, Plate, Sheet, Wire, Forging, Casting | 720 (Sol. Ann.) - 1170-1310 (H900) | 1000 (Sol. Ann.) - 1310-1450 (H900) | 196 | 7.81 | ~35 HRC (Sol. Ann.), 38-45 HRC (H900) | $$$$ |
Key Performance:
Primary Applications:Aerospace fasteners and structural components, valve components, pump shafts, gears, food processing equipment, nuclear reactor components. Critical Limitations:Loses toughness below approx. -30°C to -40°C in some heat treat conditions (e.g., H900). Optimum corrosion resistance achieved after aging. Not suitable for very high temperature service (strength drops above ~315°C / 600°F). Processing:Hardenable by precipitation aging heat treatment. Supplied in solution annealed (Condition A). Various aging treatments (e.g., H900, H1025, H1075, H1150) yield different balances of strength, toughness, and corrosion resistance. |
Aluminum Alloys
| Material | Common Equivalents | Typical Forms | Yield (MPa) | Tensile (MPa) | Modulus (GPa) | Density (g/cm³) | Hardness (HB) | Cost Tier | Details |
|---|---|---|---|---|---|---|---|---|---|
| 6061-T6 | UNS A96061, ISO AlMg1SiCu | Sheet, Plate, Bar, Rod, Tube, Pipe, Extrusion, Wire, Forging | 276 | 310 | 68.9 | 2.70 | 95 | $$ |
Key Performance:
Primary Applications:Structural extrusions (window frames, architectural components), bicycle frames, automotive components (chassis parts, suspension), marine applications (small boats, fittings), piping, scuba tanks. Critical Limitations:Strength significantly reduced in weld zones unless post-weld heat treated (re-solutionize and age). Lower strength than 2xxx or 7xxx series. Not ideal for high fatigue applications without careful design. Processing:Age-hardenable (Mg₂Si precipitates). Excellent formability in annealed (O) condition. Easily extruded into complex shapes. T6 temper involves solution heat treating and artificial aging. |
| 7075-T6 | UNS A97075, ISO AlZn5.5MgCu | Sheet, Plate, Bar, Rod, Extrusion, Forging | 503 | 572 | 71.7 | 2.81 | 150 | $$$ |
Key Performance:
Primary Applications:Aircraft structures (wing spars, fuselage frames), high-performance automotive components (connecting rods, gears), climbing gear, bicycle components, missile parts, firearm receivers. Critical Limitations:High susceptibility to SCC in T6 temper, especially in marine/humid environments. Strength degrades significantly above ~120-150°C sustained temperature. Poor weldability limits fabrication options. Processing:Age-hardenable (Zn, Mg, Cu precipitates). Limited formability in T6 condition; best formed in annealed (O) or W (solution treated) temper then aged. Overaging tempers (e.g., T73, T76) improve SCC resistance but reduce peak strength. |
| 2024-T3/T4 | UNS A92024, ISO AlCu4Mg1 | Sheet, Plate, Bar, Rod, Extrusion, Wire | 324-345 (T3/T4) | 469-483 (T3/T4) | 73.1 | 2.78 | 120 | $$$ |
Key Performance:
Primary Applications:Aircraft fuselage and wing structures (skins, tension members), rivets, truck wheels, structural components requiring good fatigue resistance. Critical Limitations:Susceptible to SCC, especially in older tempers or when improperly heat treated. Fatigue notch sensitivity requires careful design. Strength degrades significantly above ~120-150°C. Processing:Age-hardenable (Cu, Mg precipitates). Good formability in annealed (O) condition, fair in T3/T4 (T3 is solution heat treated, cold worked, and naturally aged; T4 is solution heat treated and naturally aged). Natural aging occurs at room temperature after solution treatment. |
Titanium Alloys
| Material | Common Equivalents | Typical Forms | Yield (MPa) | Tensile (MPa) | Modulus (GPa) | Density (g/cm³) | Hardness (HRC) | Cost Tier | Details |
|---|---|---|---|---|---|---|---|---|---|
| Ti-6Al-4V (Grade 5) | UNS R56400, ASTM B265/B348/B381, EN 3.7164/3.7165 | Bar, Rod, Sheet, Plate, Wire, Forging, Tube, Billet | 830-950 (Annealed) | 900-1020 (Annealed) | 110-114 | 4.43 | ~36 (Annealed) | $$$$$ |
Key Performance:
Primary Applications:Jet engine components (blades, discs, casings), aerospace fasteners and structures (airframes), medical implants (hips, knees, dental), high-performance sports equipment, marine hardware, chemical processing equipment. Critical Limitations:Hydrogen embrittlement potential above ~200-300°C or from certain chemical exposures/processing. Galvanic corrosion issues when coupled with less noble metals (e.g., aluminum, steel) without isolation. High material and processing cost. Poor wear resistance without surface treatment. Processing:Heat treatable (solution treatment and aging - STA - can significantly increase strength). Requires inert atmosphere for welding and some heat treatments above ~500°C. Difficult to cast due to high reactivity. Forging and forming require careful temperature control. |
| CP Titanium Gr2 (Commercially Pure) | UNS R50400, ASTM B265/B348, EN 3.7035 | Sheet, Plate, Bar, Rod, Tube, Pipe, Wire, Billet | 275-450 | 345-550 | 103 | 4.51 | ~82 HRB (~20 HRC equiv.) | $$$$ |
Key Performance:
Primary Applications:Chemical processing equipment (heat exchangers, tanks, piping), marine hardware, desalination plants, biomedical devices (surgical instruments, some implants), airframe components (low stress, e.g., ducting), architectural applications. Critical Limitations:Lower strength than alloys like Ti-6Al-4V. Susceptible to crevice corrosion in some reducing acids without palladium addition (Gr 7/11). Risk of ignition in pure, high-pressure oxygen environments. Strength drops significantly above ~300°C. Processing:Not heat treatable for strength. Properties primarily controlled by cold work and annealing. Readily welded and formed. Stress relief annealing may be needed after significant cold work. |
Copper Alloys
| Material | Common Equivalents | Typical Forms | Yield (MPa) | Tensile (MPa) | Modulus (GPa) | Density (g/cm³) | Hardness (HB) | Cost Tier | Details |
|---|---|---|---|---|---|---|---|---|---|
| C11000 ETP Copper (Electrolytic Tough Pitch) | UNS C11000, CW004A (EN) | Sheet, Strip, Plate, Bar, Rod, Wire, Tube, Busbar | 69 (Ann) - 365 (Hard) | 220 (Ann) - 380 (Hard) | 115-117 | 8.94 | 40 (Ann) - 110 (Hard) | $$$ |
Key Performance:
Primary Applications:Electrical conductors (wires, busbars, contacts), heat exchangers (radiators, condensers), plumbing tubes, gaskets, roofing sheet. Critical Limitations:Susceptible to hydrogen embrittlement if heated in a reducing atmosphere above ~370°C (use OFHC C10100/C10200 - Oxygen-Free High Conductivity - to avoid this). Poor strength at elevated temperatures (>200°C). Low wear resistance. Processing:Strength increased by cold work. Excellent ductility and formability. Easily joined by soldering/brazing. Annealing softens and restores ductility. |
| C36000 Free-Cutting Brass (Cu-Zn-Pb) | UNS C36000, CZ121 (BS) | Rod, Bar, Shapes (limited) | 124 (Ann) - 310 (Hard Drawn) | 338 (Ann) - 470 (Hard Drawn) | 97 | 8.50 | 65 (Ann) - 120 (Hard Drawn) | $$$ |
Key Performance:
Primary Applications:Precision machined parts (screws, nuts, bolts), fittings (plumbing, pneumatic), valve components, gears, architectural hardware, musical instrument parts. Critical Limitations:Dezincification in corrosive water. Susceptible to SCC in ammonia environments. Poor cold formability due to lead content. Lead content raises environmental/health concerns in some applications. Processing:Lead addition (~2.5-3.7%) provides excellent machinability. Primarily used for hot forming or machining from rod/bar stock. Limited cold workability. |
| C63000 Nickel-Aluminum Bronze (Cu-Al-Ni-Fe) | UNS C63000, AMS 4640, ASTM B150 | Rod, Bar, Tube, Forging, Casting, Plate (limited) | 380-550 (As cast/extruded, depends on HT) | 690-820 (As cast/extruded, depends on HT) | 117-121 | 7.53-7.58 | 170-230 (As cast/extruded) | $$$$ |
Key Performance:
Primary Applications:Marine propellers, pump impellers and bodies, valve seats and stems, bearings, gears, heavy-duty bushings, non-sparking tools, components for offshore platforms. Critical Limitations:Can be susceptible to dealuminification (selective leaching of aluminum) in some aggressive acidic or high-chloride environments if not properly heat treated or if a less resistant composition is used. Higher cost than common brasses/bronzes. Processing:Heat treatable (quenching and tempering can optimize properties). Available in cast and wrought forms (e.g., extrusions, forgings). Good hot workability. |
Tool Steels
| Material | Common Equivalents | Typical Forms | Typical Hardness (HRC) | Key Performance Chars. | Cost Tier | Details |
|---|---|---|---|---|---|---|
| O1 (Oil Hardening) | UNS T31501, AISI O1, BS BO1, JIS SKS3 | Bar, Rod, Ground Flat Stock, Drill Rod | 57-62 | Good wear resistance, fair toughness, good machinability (annealed), fair dimensional stability in HT. | $$$ |
Primary Applications:Cutting tools (short run taps, drills, reamers), gauges, blanking and forming dies for simpler shapes and lower volumes, woodworking tools. Critical Limitations:Requires oil quench which can lead to higher distortion than air hardening grades. Lower wear resistance than A2/D2. Limited toughness at maximum hardness. Max service temp ~150-200°C. Processing Considerations:Oil hardening group. Requires precise temperature control in heat treatment (austenitizing, quenching, tempering). Tempering critical for achieving desired toughness/hardness balance. Anneal for machinability. |
| A2 (Air Hardening) | UNS T30102, AISI A2, BS BA2, JIS SKD12 (approx.) | Bar, Rod, Ground Flat Stock, Plate | 58-62 | Very good wear resistance, good toughness (better balance than O1), fair machinability (annealed), good dimensional stability in HT. | $$$$ |
Primary Applications:Punches and dies (medium to high volume stamping/forming), shear blades, stamping tools for complex shapes, coining dies, long-lasting cutting tools. Critical Limitations:Lower wear resistance than D2. Requires higher austenitizing temperatures than O1. Can be more challenging to grind than O1. Max service temp ~200-250°C. Processing Considerations:Air hardening group provides less distortion than oil hardening. Requires careful heat treatment. Multiple tempers often used to optimize toughness. Surface treatments (nitriding, PVD) can further enhance wear resistance. |
| D2 (High C, High Cr Cold Work) | UNS T30402, AISI D2, BS BD2, JIS SKD11, EN X153CrMoV12 (1.2379) | Bar, Rod, Ground Flat Stock, Plate | 58-62 (can reach 64) | Excellent wear resistance (highest among common cold work tool steels), fair to moderate toughness, poor machinability (annealed), good dimensional stability in HT. Some corrosion resistance. | $$$$ |
Primary Applications:High-volume blanking and forming dies, slitting cutters, thread rolling dies, long-run stamping tools, punches, wear parts, knives requiring high edge retention. Critical Limitations:Relatively brittle compared to A2 or O1, especially if not properly heat treated (requires higher austenitizing temps and careful tempering). Difficult to grind and machine. Susceptible to chipping in shock applications. Max service temp ~200-300°C. Processing Considerations:Air hardening, can also be oil quenched in some sections but air preferred for stability. Requires careful grinding post-HT using appropriate wheels. Multiple tempers often required. Cryogenic treatment can improve wear resistance and dimensional stability. |
Superalloys (High-Performance Alloys)
| Material | Common Equivalents | Typical Forms | Yield (MPa) at RT | Tensile (MPa) at RT | Density (g/cm³) | Max Recommended Service Temp (°C) | Cost Tier | Details |
|---|---|---|---|---|---|---|---|---|
| Inconel 718 (Ni-based) | UNS N07718, AMS 5596/5662, EN NiCr19Fe19NbMo3 (2.4668) | Bar, Rod, Sheet, Plate, Wire, Forging, Tube, Casting, Powder | ~1035-1240 (Aged) | ~1240-1380 (Aged) | 8.19 | ~650-700 (for high stress) | $$$$$$ |
Key Performance:
Primary Applications:Gas turbine engine components (discs, blades, shafts, casings), aerospace fasteners, nuclear reactor components, rocket motors, cryogenic tankage, turbocharger rotors, chemical processing equipment. Critical Limitations:Extremely difficult to machine. Requires specialized processing (vacuum induction melting, electroslag remelting, controlled forging). Susceptible to strain-age cracking during post-weld heat treatment if not properly managed. High cost. Processing:Precipitation hardenable (primarily by γ'' - Ni₃Nb). Typically solution treated and aged. Welding requires specific procedures (e.g., TIG, EBW) and often post-weld heat treatment. Forging requires tight temperature control. |
| Hastelloy X (Ni-Cr-Fe-Mo) | UNS N06002, AMS 5754, EN NiCr22Fe18Mo (2.4665) | Sheet, Plate, Bar, Wire, Forging, Tube, Pipe | ~240-365 (Solution Annealed) | ~655-785 (Solution Annealed) | 8.22 | Up to ~1000-1200 (for oxidation resistance, lower for significant stress) | $$$$$$$ |
Key Performance:
Primary Applications:Gas turbine combustors and afterburner components (cans, ducting, flame holders), industrial furnace parts (muffles, retorts, radiant tubes), chemical process industry components requiring high-temp oxidation resistance and resistance to stress corrosion cracking. Critical Limitations:Not as strong as precipitation-hardened superalloys like Inconel 718 at moderate temperatures (below ~700°C). Subject to aging embrittlement (loss of ductility) after long exposures in the 650-900°C range if not carefully considered in design. Very high cost. Processing:Solid-solution strengthened (not precipitation hardenable). Typically used in the solution annealed condition. Readily welded (TIG, MIG, resistance) and formed using techniques for Ni-based alloys. Careful cleaning is essential before heating. |
Emerging Metallic Materials
AHSS are complex, sophisticated steels with carefully controlled microstructures (e.g., martensitic, bainitic, ferritic with embedded hard phases like martensite in Dual Phase - DP steels, or retained austenite in TRIP steels). They offer significantly higher strength (typically >550 MPa yield) compared to conventional steels, allowing for weight reduction in components without compromising safety or performance.
Key Characteristics:
- High strength-to-weight ratio
- Good formability for their strength level (varies by grade)
- Improved crashworthiness and energy absorption
Primary Applications:
Automotive body structures (pillars, rails, bumpers, door intrusion beams), chassis components, agricultural equipment.
Considerations:
Weldability can be challenging (requires specific procedures), springback during forming, higher cost than conventional steels.
MMCs consist of a metal matrix (e.g., aluminum, titanium, magnesium) reinforced with a secondary phase, typically ceramic particles (e.g., Silicon Carbide - SiC, Alumina - Al₂O₃) or fibers (e.g., carbon, SiC). The reinforcement enhances specific properties of the base metal.
Key Characteristics:
- Increased stiffness and strength
- Improved wear resistance
- Enhanced high-temperature performance
- Tailorable thermal expansion and conductivity
Primary Applications:
Aerospace components (structural parts, engine components), automotive parts (brake rotors, pistons, connecting rods), electronic packaging/heat sinks, sporting goods.
Considerations:
Higher cost, potentially reduced ductility and toughness compared to unreinforced matrix, complex fabrication processes, machining challenges.
Amorphous metals lack a long-range ordered crystalline structure, resulting in a "glassy" atomic arrangement. This is achieved by very rapid cooling of molten alloys.
Key Characteristics:
- Very high strength and hardness (often exceeding crystalline counterparts)
- Excellent elasticity (high elastic strain limit)
- Good corrosion and wear resistance
- Unique magnetic properties (soft or hard, depending on composition)
Primary Applications:
Transformer cores (low energy loss), sporting equipment (golf clubs, baseball bats), consumer electronics casings (watches, phones), medical implants and surgical tools, precision molds, wear-resistant coatings.
Considerations:
Limited size/thickness due to rapid cooling requirement (though BMGs - are improving this), can be brittle in tension, specialized processing, higher cost.
HEAs are a newer class of alloys typically composed of five or more principal elements in relatively equal or near-equal atomic percentages (5-35 at.% each). This high configurational entropy can lead to the formation of simple solid-solution phases (e.g., FCC, BCC) instead of complex intermetallics, offering unique property combinations.
Key Characteristics:
- High strength and hardness
- Good ductility and toughness (in some systems)
- Excellent wear and corrosion resistance
- Good thermal stability and high-temperature strength
- Potential for exceptional fatigue resistance and radiation tolerance.
Primary Applications:
Still largely in research & development, but potential uses include: high-temperature structural components (aerospace, power generation), wear-resistant coatings, cryogenic applications, biomedical implants, catalysts, nuclear reactor materials.
Considerations:
Vast compositional space makes alloy design complex, processing can be challenging, understanding long-term phase stability is ongoing, generally high material cost due to multiple (often expensive) elements.
Selection Decision Matrix
- 1. Ti-6Al-4V (aerospace, medical)
- 2. 7075-T6 Aluminum (performance automotive, aerospace)
- 3. High-Strength Alloy Steels (e.g., 4340, AHSS) (high load, when cost is a greater concern than weight vs. Ti/Al)
- 4. Magnesium Alloys (ultra-lightweight, specific applications - not detailed above but relevant)
- 5. MMCs (Al or Mg matrix) (specialized high performance)
- 1. Titanium Alloys (CP Ti, Ti-6Al-4V) (extreme environments, seawater, many chemicals)
- 2. Superalloys (Inconel, Hastelloy) (aggressive chemical and high-temp environments)
- 3. 316 Stainless Steel (marine, chemical, pharmaceutical)
- 4. Nickel-Aluminum Bronze (seawater, anti-fouling)
- 5. 6061 Aluminum (atmospheric, fresh water)
- 6. Amorphous Metals (some compositions) (excellent in specific media)
- 1. Superalloys (e.g., Hastelloy X, Inconel 718) (>650°C, up to 1200°C for some)
- 2. Some Stainless Steels (e.g., 310S, specialized grades) (500-800°C, depends on grade and atmosphere)
- 3. Refractory Metals (Mo, W, Ta) (>1200°C - not detailed above but critical for extreme temps)
- 4. Tool Steels (Hot Work Grades like H13) (Up to ~500-600°C with tempering considerations)
- 5. High Entropy Alloys (some compositions) (potential for very high temps)
- 1. A36 Carbon Steel (structural, low stress, lowest cost)
- 2. 6061 Aluminum (moderate strength, good corrosion resistance, light weight, good processability)
- 3. 304 Stainless Steel (good corrosion resistance, aesthetic appeal, moderate cost)
- 4. Medium Carbon Alloy Steels (e.g., 4140) (higher strength than plain carbon, heat treatable, moderate cost)
- 1. Copper Alloys (e.g., C11000 ETP) (highest electrical/thermal)
- 2. Aluminum Alloys (e.g., 6061, 1xxx series) (very good electrical/thermal, lighter than copper)
- 3. Carbon Steels (moderate thermal conductivity, poor electrical)
- 4. MMCs (e.g. Al/SiC for thermal management) (tailorable)
- 1. Tool Steels (D2, A2, O1 - hardened) (dies, cutters, wear parts)
- 2. Hardened Alloy Steels (e.g., 4140, 4340 - nitrided or case hardened)
- 3. Nickel-Aluminum Bronze (C63000) (bearings, gears)
- 4. Amorphous Metals (Metallic Glasses) (coatings, precision parts)
- 5. MMCs (with ceramic reinforcement) (specialized wear components)
- 6. High Entropy Alloys (some compositions)
Processing Compatibility Warning Matrix
- ⚠️ Welding Difficulties/Restrictions:
- 7075 & 2024 Aluminum: Generally not recommended for fusion welding (prone to cracking).
- Tool Steels (Hardened): Require special procedures (pre/post heat, specific consumables) if welded at all; often avoided.
- Martensitic Stainless Steels (e.g., 400 series hardened): Require pre/post heat.
- Titanium Alloys: Require inert gas shielding for all heated zones to prevent contamination.
- Superalloys: Often require specialized techniques, consumables, controlled atmospheres, and are prone to cracking.
- Some AHSS: Can have narrow welding windows and HAZ (Heat Affected Zone) softening concerns.
- ⚠️ Galvanic Corrosion Risk - Dissimilar Metal Contact:
- Aluminum + Steel/Stainless Steel/Copper: Aluminum will corrode preferentially. Isolation required.
- Titanium + Steel/Aluminum: Steel/Aluminum will corrode. Isolation often needed.
- Carbon Steel + Stainless Steel: Carbon steel corrodes.
- Always consult a galvanic series chart for specific environment and potential difference.
- ⚠️ Hydrogen Embrittlement Risk:
- High-Strength Steels (e.g., 4340, Tool Steels, some AHSS): Susceptible, especially after plating, pickling, or in hydrogen-rich environments. Baking after plating is crucial.
- Titanium Alloys: Can absorb hydrogen at elevated temperatures or from certain processes, leading to embrittlement.
- Martensitic Stainless Steels: Can be susceptible.
- ⚠️ Critical Heat Treatment Requirements:
- All heat-treatable alloys (Alloy Steels, PH Stainless, Al Alloys (2xxx, 6xxx, 7xxx), Ti Alloys, Tool Steels, Superalloys) require precise temperature control, soak times, and quench/aging parameters to achieve desired properties. Deviations can lead to drastically reduced performance or failure.
- Austenitic Stainless Steels (304, 316): Can be sensitized (loss of corrosion resistance at grain boundaries) if heated in ~450-850°C range (e.g., during welding without L-grade or stabilization).
- ⚠️ Machinability Challenges:
- Titanium Alloys: Low thermal conductivity, galling, work hardening, reactivity.
- Superalloys: Extreme work hardening, high strength at cutting temps, abrasive phases.
- Austenitic Stainless Steels: High work hardening rate, gummy chips.
- Tool Steels (Hardened): Very difficult, often requires grinding or specialized hard machining.
- MMCs: Abrasive reinforcements cause rapid tool wear.
- ⚠️ Stress Corrosion Cracking (SCC) Susceptibility:
- High-Strength Al Alloys (7075, 2024): Especially in certain tempers (e.g., T6 for 7075) and corrosive environments (chlorides). T7x tempers improve resistance.
- Austenitic Stainless Steels (304, 316): In chloride environments above ~60°C under tensile stress.
- High-Strength Steels: In specific corrosive media under tensile stress.
- Brasses (High Zinc): In ammonia environments (season cracking).